Most should be taken on empty stomach. Most should be refridgerated and bought from reliable health stores that care to keep them that way. However, many store bought brands do nothing for me. In general, there are multi-strain and single strain products out there. The better ones are usually more than 8 billion per cap, generally speaking. Many probiotic strains also help regulate mood and keep anxiety at bay while boosting/balancing the immune system by crowding out the opportunistic co-infections (staph/ecoli/pathogens of all manner) that take up residence in inflammed tissue with good bacteria populations.
The brands/strains I've liked best, and noticed direct benefit from (mind you I have been re-sectioned near the terminal ileum 8+ years ago), are: Jarrow EPS (2-4 a day), Garden of Life Primal Defense Ultra (2-4), Thorne Research Bacillus Coagulans (2), Living Streams Mission liquid probiotic (L. Paracasei, follow instructions), Attune candy bars (or Howaru strains in other brands' capsules), Goodbelly drinks (or L. Plantarum probios), the NOW Acidophilus/Bifidus 8 bill per cap ones seem to work okay...
You can find many of these at like a Wholefoods, better independent health store, or online at mega vitamin sites (iherb,vitacost,amazon,luckyvitamin etc...).
I would be surprised if these didn't noticeably help the majority of Crohnnies to a directly observable outcome. If not, you can be assured that these help prevent further damage and regulate mood.